Dumbbell



FIG. 1 is a perspective view of a dumbbell showing my new design;

FIG. 2 is a top view thereof;

FIG. 3 is an end view thereof, the opposite side being a mirror image; and,

FIG. 4 is a side view thereof, the opposite side being a mirror image. 

The ornamental design for a dumbbell, as shown and described. 